Intershaft seals and intershaft seal assemblies may be used to isolate spaces between shafts in turbine engines having co-axial shafts. In one common design, a first shaft connects a fan, a first stage compressor, and a second stage turbine while a second shaft connects a second stage compressor and first stage turbine. The first shaft rotates at a relatively lower speed than the second shaft. The first and second shafts are co-axial and may be either co- or counter-rotational. To be effective, an intershaft seal must therefore isolate spaces between the shafts having potentially high differential rotational speeds, and the spaces may also have a potentially high differential pressure.
Intershaft seals are used in turbine engines which provide energy for a wide range of uses. Examples of turbine engines include turbofan, turbojet, turboshaft, and turboprop engines. As just one example of the wide range of applications such engines are suitable for, gas turbine engines are used to provide propulsion to an aircraft.
A typical gas turbine engine comprises an inlet fan, a compressor, a combustor, a high-pressure turbine, and a low-pressure turbine. As one example of a typical dual-shaft gas turbine engine 50, FIG. 1 illustrates a first shaft 20 which connects a fan 52, first stage compressor 54, and second stage turbine 62. A second shaft 24 is hollow and is concentrically located around first shaft 20 and connects a second stage compressor 56 with a first stage turbine 60. A combustor 58 is disposed between second stage compressor 56 and first stage turbine 60. First shaft 20 is radially inward from second shaft 24 and rotates at a relatively lower speed. Intershaft seal assemblies 10 are used at least at each axial terminus of outer shaft 24 to seal the spaces between the two concentric shafts 24, 20.
One design for an intershaft seal involves the use of a seal ring which is sometimes referred to in the art as a piston ring. FIG. 2 illustrates a seal ring design for a prior art intershaft seal. Intershaft seal assembly 10 comprises a seal ring 12 in contact with an annular retaining arm 14. The seal ring 12 is disposed between a pair of runners 16 (or retaining rings) which are spaced apart by a spacer 18 and coupled to an inner shaft 20. Retaining arm 14 is coupled to a hollow outer shaft 22 and may be held in place by a retention member 24. Inner shaft 20 and outer shaft 24 can be co- or counter-rotational. Seal assembly 10 serves to isolate high pressure fluid cavity 30 from a lower pressure fluid cavity 32.
When inner shaft 20 and outer shaft 24 are not in motion, a slight gap (not shown) is present between seal ring 12 and retaining arm 14. However, once inner shaft 20 begins to rotate the centrifugal force from rotation will move seal ring 12 radially outward and into contact with retaining arm 14. Seal ring 12 and runners 16 are initially each rotating in the same direction and at the same rotational speed as inner shaft 20. Once seal ring 12 is in contact with retaining arm 14, seal ring 12 will begin rotating in the same direction and at substantially the same rotational speed as outer shaft 24.
FIG. 3 illustrates some of the forces acting on seal ring 12 during operation of the turbine engine (i.e. while inner shaft 20 and outer shaft 24 are rotating). A relatively large centrifugal force (Fcentrifugal) from rotation of the inner shaft 20 acts on seal ring 12 in a radially outward direction, bringing seal ring 12 into contact with retaining arm 14. An axial differential pressure force (FD/P) acts on seal ring 12 in the vicinity of the pressure boundary in a direction from high pressure fluid cavity 30 to low pressure fluid cavity 32. To form an effective seal, the centrifugal force must be large enough to hold seal ring 12 in contact with retaining arm 14 despite the axial force of differential pressure across the seal ring 12.
Forces caused by relative lateral motion (Flateral movement) between the inner shaft 20 and outer shaft 24 act on seal ring 12 in a direction either axially forward or axially aft. Finally a moment M, sometimes referred to as ring tension, resists radial expansion during rotation of seal ring 12.
The configuration described above with reference to FIGS. 2 and 3 has drawbacks. The difference in rotational speeds between inner shaft 20 and outer shaft 24 creates high friction between seal ring 12 (rotating with outer shaft 24) and runners 16 (rotation with inner shaft 20) during transients when the forces caused by relative lateral movement between the shafts overcomes the centrifugal force effecting contact between seal ring 12 and the outer shaft retaining arm 14 thus forcing the seal ring 12 to contact the forward or aft runner 16. This high friction can cause excessive heat generation in the seal assembly 10 as well as a high wear rate of seal ring 12.
Thus there is a need in the art for an effective intershaft seal assembly which is better suited to resist heat generation and wear of the seal ring.
While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims.